It is widely believed among physicists that the theory of quantum gravity, which aims to unify quantum mechanics and general relativity, will indeed be more complex than Einstein's theory of general relativity, just as general relativity is more complex than Newtonian mechanics.
Einstein's theory of general relativity revolutionized our understanding of gravity by describing it as the curvature of spacetime caused by mass and energy. It introduced new mathematical concepts and required a different way of thinking about gravity compared to Newton's theory of gravitation. General relativity successfully explains various phenomena, such as the bending of light around massive objects and the expansion of the universe.
However, general relativity is a classical theory and does not incorporate quantum mechanics, which is the framework that describes the behavior of matter and energy on extremely small scales. Quantum mechanics, on the other hand, deals with particles and their interactions in a probabilistic and non-deterministic manner.
The challenge lies in reconciling these two theories into a single framework known as quantum gravity. This theory aims to provide a consistent description of gravity at both macroscopic (general relativity) and microscopic (quantum mechanics) scales. Currently, there is no widely accepted theory of quantum gravity, but several approaches, such as string theory, loop quantum gravity, and causal dynamical triangulations, are actively being studied.
The search for a theory of quantum gravity requires addressing deep conceptual and mathematical challenges. It involves understanding the behavior of spacetime at extremely small distances and high energies, where the effects of quantum mechanics and gravity become intertwined. This unification may involve new mathematical structures, novel physical concepts, and a more intricate framework than general relativity.
While we cannot say with certainty how complex the theory of quantum gravity will be until it is fully developed, it is reasonable to expect that it will be more intricate and mathematically involved than general relativity, just as general relativity expanded upon Newtonian mechanics.